Preheat oven to 350F. Mist a medium casserole dish with nonstick cooking spray and set aside. Heat oil in a large skillet. When hot add onion and garlic and saute until onion is well browned and very tender. Meanwhile, combine tomato paste, tamari, molasses or maple syrup, and mustard in a large bowl. Mix well to form a smooth, thick sauce. Add beans and mix gently. Stir cooked onion into beans and spoon mixture into prepared casserole dish. Cover tightly and bake for 30 minutes.